The design of tennis rackets, which stood still for fifty years, has been developing rapidly in the last ten years. The spread of tennis, the greater number of players and the higher quality of those players, have led to problems with equipment. These problems have been aggrevated by the transfer of the production of rackets to countries with low labour force costs and by the general use of new forms of frame allowed by the absence of regulations.
The undustrialisation of the manufacture of wooden rackets means that there is now a less careful selection of the tree trunks used, natural drying has been replaced by steam drying, wood peeling has come into use instead of assembling leaves of wood cut in the direction of the grain, and the result is a lower performance from the rackets produced. Gluing, machining and drilling by semi-automatic methods cannot achieve a quality equal to the quality attainable by craftsmen.
The known idea of a frame made from a metal element bent to a curved portion forming the racket head and two arms forming the racket shaft is reliable only if tempered steel is used with a brazed join between the arms. However, when using a racket having such a frame it is necessary to centre the ball perfectly or vibrations are set up which are prejudicial to the player. Such a metal frame is therefore thought applicable only to average-quality frames.
"Sandwich" structures with a lining parallel to the stringing plane and used on a plastic neutral fibre or as reinforcement for a wooden structure are subjected to shearing stress owing to the dynamic or static stresses of the stringing. They improve the bending strength of the frames but present adhesion problems. The cutting-out of impregnated fabrics does not permit suitable orienting of the fibres and results in prohibitive loss of material. The possible improvement of linings cannot eliminate the problems of the shearing stress. Reinforcements made of metal or laminate material perpendicular to the stringing plane are not of significant value because of the necessary perforations for the strings and the usually central positioning of these reinforcements.
It is also known to mould racket frames from continuous glass fibres or carbon fibres impregnated with thermosetting epoxy resins. However, such methods involve a long production process and hence a considerable cost. Rackets with suitably oriented fibres are reserved for the elite.
The employment of thermoplastic materials which may be processed rapidly makes it possible to reduce the number of operations required for the production of a frame. The choice of the quality and the addition of cut fibres makes it possible to obtain an anisotropic material of high modulus of elasticity endowed with excellent return characteristics. Polyamides with carbon fibres as filler, forming a mixture, are used:
with simple injection moulding: here the density of the mixture makes it necessary to reduce the cross-sections of the frame, hence there is a diminishing of the moment of inertia of the frame; PA1 with injection moulding of a mixture lightened by a swelling agent (U.S. Pat. No. 3,981,504 assigned to P.P.G. Industries): here the loss in bending properties is not made up for by the increase in cross-sections that can be achieved; PA1 with injection moulding on to a fusible core (U.S. Pat. No. 4,297,308 assigned to Dunlop): this affords the advantage of a hollow structure with central reinforcements and an integral join between arms, but requires an extremely complex production process for its manufacture; PA1 with assembly of injection moulded elements resulting in a continuous internal rib (U.S. Pat. No. 4,194,738 assigned to Hitachi).
The use of injection moulding normally limits the amount of carbon admixed in the mixture which inhibits exploiting all the properties of the material. During injection moulding a sheath of thermoplastic material in contact with the mould modifies the distribution of the fibres and the characteristics of the mixture. The injection pressure and the movement of the mixture through passages of small cross-section causes fracture of the fibres, such that their residual length does not allow the theoretical characteristics of the mixture to be maintained. The shape and dimensions of the frame require material flow welding, and at the head of the racket a local loss of strength is also observed between the perforations. Despite these disadvantages which, added to design faults, result in a racket with inadequate rigidity, solid-section rackets injection moulded from non-lightened materials have excellent return characteristics, confirming the usefulness of the material.